1. Field of the Invention
The present invention relates to a piezoelectric actuator including a piezoelectric film sandwiched between two electrode layers and a diaphragm, and a liquid discharge head using the piezoelectric actuator.
2. Description of the Related Art
In a piezoelectric actuator including a piezoelectric film sandwiched between two electrode layers and a diaphragm, when an electrical field is applied between the two electrode layers, the piezoelectric film expands and contracts in a film thickness direction and an in-plane direction. A piezoelectric actuator, which has such a structure that the diaphragm is bent by expansion and contraction of the piezoelectric film in the in-plane direction between the expansion and contraction modes of the piezoelectric film which can be divided into two directions described above, is normally called a bending mode type piezoelectric actuator. The piezoelectric actuator having the above-mentioned structure is widely used because a bending amount of the diaphragm can be obtained as a displacement.
When the piezoelectric film has a film thickness of several μm, the piezoelectric actuator having the above-mentioned structure can be used as, for example, a piezoelectric actuator for an ink jet head.
As a method of forming the piezoelectric film having the above-mentioned film thickness, there is normally used a method of applying, onto the diaphragm, a solution containing a precursor of a piezoelectric material serving as the piezoelectric film and an organic binder by a screen printing method, and then performing drying and baking to form the piezoelectric film. In addition, there is widely used, for example, a method of forming, on the diaphragm, a green sheet including the precursor of the piezoelectric material serving as the piezoelectric film and the organic binder, and then performing baking to form the piezoelectric film.
In recent years, for example, in a case of an ink jet printer onto which an ink jet head is mounted as a piezoelectric actuator utilizing the displacement of the bending mode, the ink jet head is required to have a higher speed and higher image quality.
Therefore, the ink jet head is required to enable precise displacement drive control and stably discharge minute liquid droplets of ink, and to have higher displacement power in order to discharge high-viscosity ink and also in order to increase a density of discharge ports.
In order to realize performance required for the ink jet head, a piezoelectric film whose crystal structure is controlled to obtain high piezoelectric performance has been developed. Such a piezoelectric film is formed by the following film formation methods which are generally known as thin film formation methods.                Sputtering film formation method        Metal organic chemical vapor deposition (MOCVD) film formation method        Pulse laser deposition (PLD) film formation method        Molecular beam epitaxy (MBE) film formation method        Chemical solution deposition (CSD) (=sol-gel method) film formation method        
According to the above-mentioned film formation methods, the piezoelectric film can be directly formed on a target, and thus the film formation methods can be used for minute processing using a semiconductor manufacturing process. Therefore, attention has been focused on the film formation methods as effective methods of manufacturing a piezoelectric actuator having a minute structure, such as a micro electro mechanical system (MEMS).
For example, Japanese Patent Application Laid-Open No. 2004-042287 discloses a liquid discharge head including a piezoelectric element including a lower electrode, a piezoelectric layer, and an upper electrode which are provided, through a diaphragm, on a surface side of a substrate serving as a flow path of an ink jet head. In the liquid discharge head, a zirconium oxide layer, a cerium oxide layer, and a superconductor layer made of an yttrium-barium-copper-oxide-based material (YBCO) are provided on the substrate. The lower electrode made of strontium ruthenate and formed on the superconductor layer, the piezoelectric layer formed on the lower electrode, and the upper electrode formed on the piezoelectric layer are also provided. Japanese Patent Application Laid-Open No. 2004-042287 discloses that a Pb(Zr,Ti)O3 (PZT) film is formed on the lower electrode by the sol-gel method to realize single crystallization of a PZT crystal structure, and the piezoelectric layer having (100) orientation is obtained, whereby liquid discharge with a uniform piezoelectric characteristic can be performed at maximum output. Further, Japanese Patent Application Laid-Open No. 2004-042287 discloses in the second and third aspects, when the crystal plane orientation of the piezoelectric layer is (100) orientation in the above-mentioned structure, the piezoelectric characteristics can be substantially improved.
A piezoelectric actuator, in particular, a piezoelectric actuator utilizing the bending mode in a case where a thin film formation method is employed has a structure in which a distortion of the piezoelectric film is converted into a bending of the diaphragm. Therefore, in order to bring out the substantial performance such as the bending amount in the piezoelectric actuator including the diaphragm and the piezoelectric film sandwiched between the two electrodes, not only the distortion performance of the piezoelectric film, but also a relative relationship between the piezoelectric film and the diaphragm are extremely important. That is, in order to effectively use the excellent piezoelectric characteristics of the piezoelectric film for the piezoelectric actuator having the bending mode, it is important to design the piezoelectric actuator in view of the relative relationship between the piezoelectric film and the diaphragm, which substantially determines the performance of the piezoelectric actuator. In particular, it is important to make compatible a displacement amount of the piezoelectric actuator with a response of the piezoelectric actuator to a driving waveform thereof.